LIBRARY OF CONGRESS 




0017 111 5140 • 




STATES DEPARTMENT OF AGRICULTURE 

.ru^,.. BULLETIN No. 552 



Contribution from the Forest Service 
HENRY S. GRAVES, Forester 




SU^^-mTU 



Washington, D. C. 



PROFESSIONAL PAPER 



July 9, 1917 



THE SEASONING OF WOOD/ 

By Harold S. Betts, M. E., in charge, Office of hidustrial Investigations. 



CONTENTS. 



Page. 

Importance of proper seasoning methods 1 

Fiber saturation point and shrinkage 2 

How wood may be injured in seasoning 11 

Checking 11 

Casehardening 11 

Honeycombing 11 

Warping 12 

Collapse 12 



Page. 

Air seasoning 12 

Crossties, poles, and sawed timbers 12 

Lumber 17 

Rules for piling lumber 20 

Kiln-drying 22 

Types of kilns 22 

Preliminary treatments 25 

The process of drying 26 



IMPORTANCE OF PROPER SEASONING METHODS. 

Practically all wood before being put to use is either seasoned in tbe 
air or dried in a kiln. The main objects of seasoning are to increase 
the durability of the wood in service, to prevent it from shrinking 
and checking, to increase its strength and stiffness, to prevent it 
from staining, and to decrease its weight. The sooner wood is sea- 
soned after being cut the less is the chance that it wall be injured by 
I the insects, which attack unseasoned wood,^ or decay before the time 
comes to use it. Wood that is to be treated with preservatives needs 
in nearly all cases to be seasoned as much as wood that is to be used 
in the nat/ural state. 

Wood has a complicated structure. The walls of the cells of 
which it is made up shrink and harden when moisture is removed 
from them, and unless timber that is to be air-seasoned is piled in 
the right way, or conditions in the dry kiln are maintained in accord- 
ance with certain well-defined physical laws, the material is hkely to 
warp or check, or in some way to be damaged seriously. Until 
recently proper methods of seasoning received comparatively little 
attention from manufacturers, and large losses, especially among 

1 For assistance and si^gestions given in connection with the preparation of this bulletin, the author is _ 
I indebted to Mr. D. P. Sexton, of John B. Ransom & Co., Nashville, Tenn., and to Messrs. R. K. Helphen^ 

I Btine, jr., and N. de AVitt Betts, of the Forest Service. 

2 The sapwood of seasoned hardwood is subject to attack and frequently to serious damage by powder. 
! post insects. See Farmers' Bulletin 778, "Powder-Post Damage by Lyctus Booties to Seasoned Hard- 
I wood," by A. D. Hopkins and T. E. Snyder, 1917. 

87732°— BuU. 552—17 1 ""„ ., 



s 

2 BULLETIISr 552. U. S. DEPARTMENT OF AGRICULTUReA ^C 

woods that are difficult to dry, were the rule. Sometimes as mucn 
as 20 or 25 per cent of the seasoned himber was rendered unfit for the 
use intended by defects which had their origin in the drying process. 
Since the quality of the finished product can be impaired seriously by 
wrong methods, the importance of right methods becomes apparent. 

FffiER SATURATION POINT AND SHRINKAGE. 

Water exists in wood in two conditions:^ (a) as free water con- 
tained in the cell cavities, and (b) as water absorbed in the cell walls. 
When wood contains just enough water to saturate the cell walls, it 
is said to be at the ''fiber saturation point." Any water in excess of 
this which the wood may contain is in the form of free water in the 




Fig. 1.— Shrinkage as aSected by direction of annual rings; approximately twice as great tangentially 

as radially. 

cell cavities. Removal of the free water has no apparent effect upon 
the properties of the wood except to reduce its weight, but as soon 
as any of the absorbed water is removed the wood begins to shrink. 
Since the free water is the first to be removed, shrinkage does not 
begin, as a general rule, until the fiber saturation point is reached. 
In the case of eucalyptus and some of the oaks, however, shrinkage 
begins above this point. For most woods the fiber saturation point 
corresponds with a moisture content of from 25 to 30 per cent of the 
dry weight of the wood. Figure 1 shows graphically the difference 
between tangential and radial shrinkage. 

Shrinkage is due to the contraction of the cell Myalls, and sets up 
stresses which tend to cause the wood to check. As observed in a 
cross section of a piece of lumber, shrinkage in the tangential direc- 
tion is about twice as great as in the radial direction; lengthwise of 

' The term "sap" sometimes is used wrongly to mean the moisture in wood, and at other times to mean 
the sapwood. Sap is formed, mainly in the early spring, in the leaves from water rising from the roots 
through the sapwood . In the leaves this water is converted into true sap, which contains sugar and soluble 
gums. The sap descends through the bark and feeds the tissues in process of formation between the bark 
and the sapwood. The heartwood contains no sap. 



//- /// 



D. : of D. 
J^L 23 1917 



THE SEASONING OF WOOD. 



the lumber it is very slight. Table I gives the green, air-dry, and 
kiln-dry weight per cubic foot of the principal commercial woods, 
and Table II gives the per cent of shrinkage from a green to an oven- 
dry condition. 

Table I. — Average weights of various species of wood. 



Species and locality. 



HARDWOODS. 



Alder, red, Snohomish County, Wash 

Ash: 

BUtmore, Overton County, Tenn 

Black- 
en tonagon County, Mich 

Marathon County, Wis , 

Blue, Bourbon County, Ky 

Green- 
Richland Parish, La 

New Madrid County, Mo 

Oregon, Lane County, Oreg 

Pumpkin, New Madrid County, Mo. . 
White— 

Stone County, Ark 

Oswego County, N. Y 

Pocahontas County, W. Va 

Aspen : 

Rusk County, Wis 

Largetooth, Sauk County, Wis 

Basswood: • 

Potter County, Pa 

Marathon County, Wis 

Beech: 

Hendricks County, Ind 

Potter County, Pa 

Birch: 

Paper, Rusk County, Wis 

Sweet, Potter County, Pa 

Yellow- 
Potter County, Pa. 

Marathon County. Wis 

Buckeye, yellow, Sevier Cfounty, Tenn . . . 
Buckthorn, cascara. Lane County, Oreg. . 
Butternut: 

Sauk County, Wis 

Sevier County, Tenn 

Cherry: 

Black, Potter County, Pa 

Wild red, Sevier County, Term 

Chestnut: 

Baltimore County, Md 

Sevier County, Tenn 

Chinquapin, western. Lane County, Oreg. 
Cottonwood : 

Pemiscot County, Mo 

Black, Snohomish County, Wash 

Cucumber tree, Sevier County, Tenn 

Dogwood: 

Flowering, Sevier County, Tenn 

Western, Lane County, Oreg 

Elder, pale, Douglas County, Oreg 

Elm: 

Cork- 
Marathon County, Wis 

Rusk County, Wis 

Slippery — 

Hendricks County, Ind 

Sauk County, Wis , 




Weights (pounds per cubic 
root). 



V) 



Air-dry.2 



(0 



i') 



28 



{*) 



Green.' 



(') 



47 
51 
46 

47 
43 

41 
41 

56 
54 

51 
59 

56 
59 



45 

47 

46 
33 

53 
56 
61 



(<) 



46 
50 

65 
55 
65 



• About 8 per cent moisture. 

'About 12 or 15 per (»nt moisture. Average condition reached without artificial heating by material 
sheltered from precipitation, North Central States. maowiai 

3 Average green material. 

* No figures available. 

Note.— Any individual lot of lumber in the condition specified in the column headings would proba- 
bly vary 5 per rent from the figures given with a possible variation of as much as 20 per cent. For exam- 
ple, young thrifty pmes wiU have a high moisture content when freshly cut and will probably weieh 20 oer 
cent above the average given. ' ^ ,, 6« *« pci 



4 BULLETIN 552, U. S. DEPARTMENT OF AGRICULTURE, 

Table I. — Average weights of various species of wood — Continued. 



Species and locality. 



Weights (pounds per cubic 
foot). 



Kiln- 
dry. 




Green. 



HARDWOODS— continued. 
Elm— Continued. 
White- 
Potter County, Pa 

Marathon Coimty , Wis 

Gum: 

Black, Sevier County, Tenn 

Blue, Alameda County, Cal ■. 

Red— 

New Madrid County, Mo 

Pemiscot County, Mo 

Hackberry: 

Hendricks County, Ind 

Sauk County, Wis 

Haw, pear, Sauk County, Wis 

Hickory: 

Shellbark— 

Sardis, Miss 

Napoleon, Ohio 

Bittemut, Napoleon, Ohio 

Mockemut— 

Sardis, Miss 

Chester Coimty, Pa 

Webster County, W. Va 

Nutmeg, Sardis, Miss 

Pignut — 

Sardis, Miss 

Napoleon, Ohio 

Chester County, Pa 

Webster County, W. Va 

Shagbark — 

Sardis, Miss 

Napoleon, Ohio 

Chester County, Pa 

Webster Coimty, W. Va 

Water, Sardis, Miss 

Holly, American, Sevier Coimty, Tenn 

Hornbeam, Rusk County, Wis 

Laurel: 

California, Douglas County, Oreg 

Mountain, Sevier County, Term 

Locust: 

Black, Sevier County, Tenn 

Honey, Hendricks County, Ind 

Madroiia: 

Butte County, Cal - 

Douglas County, Oreg 

Maple: 

Broadleaf, Snohomish Coimty, Wash 

Red- 
Marathon, Wis 

Potter County, Pa 

Silver, Sauk County, Wis 

Sugar— 

Hendricks County, Ind 

Potter County, Pa 

Marathon, County, Wis 

Magnolia (evergreen), Winn Parish, La 

Oak: 

Bur, Sauk County, Wis 

California black— 

Butte County, Cal 

Douglas County, Oreg 

Canyon five, Butte Coimty, Cal 

Chestnut, Sevier Coimty, Tenn 

Cow, Winn Parish, La 

Laurel, Winn Parish, La 

Pacific post, Douglas County, Oreg 

Post- 
Stone County, Ark 

Winn Parish, La 

Red- 
Stone Coimty, Ark 

Hendricks Coimty, Ind 

Richland Parish, La 

Sevier County, Tenn 



(•) 
(') 



(') 



(') 



C) 



(') 



0) 



(') 



0) 



(') 



38 



54 


56 


45 


46 


48 


50 


45 


47 




(0 


46 


47 


47 


49 


43 


45 


42 


44 


4S 


50 


41 


42 



(') 



(') 



(•) 



(') 



I No figures available. 



THE SEASONING OP WOOD. 5 

Table I. — Average weights of various species of wood — Continued. 



Species and Jocality. 



Weights (pounds per cubic 
foot). 




Green. 



HARDWOODS— continued. 
Oak — Continued. 
Spanish— 

Lowland, Winn Parish, La 

Highland, Winn Parish, l>a 

Swamp wiiite, Hendricks County, Ind 

Tanbark, Willits, Cal 

Water, Winn Parish, La 

White- 
Stone County , Ark 

Hendricks County, I nd 

Richland Parish, La 

Winn Parish, La 

Willow, Winn Parish, La , 

Yellow- 
stone County, Ark 

Marathon County , Wis 

Osage orange, Morgan County, Ind 

Poplar, yellow, Sevier County, Term 

Rhododendron, great, Sevier County, Ttnn 

Sassafras, Sevier County, Term 

Serviceberry, Sevier County, Tenn 

Silverbell tree, Sevier County, Tenn 

Soarwood, Sevier County, Term 

Sumach, staghorn, Sauk County, Wis 

Sycamore: 

Hendricks County , Ind 

Sevier County, Tenn , 

Tupelo, St. John the Baptist Parish, La 

Umbrella, Fraser, Sevier County, Tenn 

Walnut, black, Kentucky 

Willow: 

Western black, Douglas County, Greg 

Black, Sauk County , Wis '. 

Witch hazel, Sevier County, Term , 



CONIFERS. 

Cedar. 

Incense — 

Lane County, Greg 

Weed, Cal '.'.''.'."." '."■■■ 

Port Orford , Douglas County , Greg 

Western red — 

Missoula County, Mont 

Snohomish County , Wash 

White, Shawano County, Wis 

Cypress: 

Bald, St. John the Baptist Parish, La 

Yellow, Lane County, Greg '. 

Douglas fir: 

Plumas County, Cal 

Humboldt Coiinty, Cal ]..[[[[.[..... 

Johnson County, "Wyo 

Lane County, Greg ' . " 

Chehalis County, Wash 

Lewis County, Wash 

Washington and Oregon 

Missoula County, Mont 

Fir: 

Amabilis — 

Dee, Greg 

Snohomish County , Wash 

.\lpine, Grand County, Colo 

Balsam, Rusk County, Wis 

Grand- 
Missoula County, Mont 

Douglas County, Greg 

Noble, MultnomahCouuty, Greg 

White, Madera County, Cal 

Hemlock: 

Sevier County, Tenn 

Marathon County, Wis 

Mountain, Missoula County, Mont ......[............ 

Western— 

Chehalis County, Wash 

Grays Harbor and Buckley, Vvasii ................. 

' No figures available. 



47 


49 


40 


42 


50 


52 


4:i 


44 


43 


45 


46 


48 


46 


47 


46 


48 


46 


47 


45 


46 


43 


45 


40 


42 


54 


56 


27 


28 


39 


40 


31 


32 


52 


54 


31 


32 


39 


40 


32 


34 


34 


35 


35 


36 


35 


37 


30 


31 


41 


44 


30 


31 


25 


26 


45 


46 



(■) 
(') 
30 


31 


21 
23 
21 


22 
24 
21 



(') 



33 



(') 



(') 



(') 



(') 



30 



(') 



(') 



31 



31 


32 


35 


36 


31 


32 


35 


37 






26 


27 


22 


23 


24 


25 


28 


29 


27 


^'^8 


25 


26 


31 


32 


24 


25 


30 


32 



(') 



67 
62 
69 
66 
63 

59 
61 
67 
63 

76 

63 
62 
62 
38 
62 
4t 
61 
44 
53 
41 

51 
53 
66 
47 
58 

51 
51 

59 



(') 



(') 



(') 



0) 



52 
36 
28 
45 



(') 



(') 



40 



6 BULLETIN 552, U. S. DEPARTMENT OF AGRICULTURE. 

Table I. — Average lueights of various species of ivood — Continued. 



Species and locality. 



Weights (pounds per cubic 
foot). 



Kiln- 
dry. 



1- 


Air-dry. 


37 


39 


33 


34 


29 


30 


27 


28 


37 


39 


27 


28 


27 


28 


29 


30 


28 


29 


27 


28 


42 


44 


42 


"« 


39 


41 


38 


40 


35 


36 


38 


40 


32 


34 


35 


36 


43 


"« 


26 


27 


36 


37 


29 


30 


25 


26 


28 


29 


28 


29 


27 


28 


26 


27 


23 


24 


26 


27 


22 


23 


24 


25 


28 


29 


27 


28 




(') 


25 


26 


29 


30 


37 


38 


43 


45 



Green. 



CONIFERS— continued 
Larch, western: 

Missoula County, Mont 

Stevens County, Wash 

Pine: 

Jack, Barron County, Wis 

Jeffrey, Plumas County, Cal 

Loblolly, Nassau County, Fla 

Lodgepole— 

Grand County, Colo 

Gallatin County, Mont 

Granite County, Mont 

Jeflferson County, Mont 

Johnson County, Wyo 

Longleaf— 

Nassau County, Fla 

Bogalnsa, La 

Lake Charles, La 

Tangipahoa Parish, La 

Hattiesburg, Miss 

Pitch, Sevier County, Tenn 

Pond, Nassau County, Fla 

Norway, Shawano County, Wis 

Shortleaf— 

Malvern, Ark 

Bogalusa, La 

Slash, Nassau County, Fla 

Sugar, Madera County, Cal 

Table-mountain, Sevier County, Tenn 

Western white, Missoula County, Mont 

Western yellow — 

Coconino County, Ariz 

Madera County, Cal 

Douglas County, Colo 

Missoula County, Mont 

White, Shawano County, Wis 

Redwood: 

Humboldt County, Cal 

Mendocino County, Cal 

Spruce: 

Engelmann— 

San Miguel County, Colo 

Grand County, Colo 

Red- 
Coos County, N. H 

Sevier County, Tenu 

Sitka, Chehalis County, Wash 

White- 
Coos County, N. H 

Rusk County, Wis 

Tamarack, Shawano County, Wis 

Yew, western, Snohomish County, Wash 



(') 



(0 



0) 



51 

42 

60 
47 
54 

33 
47 
41 
39 
37 



(1) 



0) 



0) 



45 
54 
42 
54 
49 
42 

45 

53 
50 
54 
39 

44 
53 
49 
51 
39 

38 
39 



48 
30 



' No figures available. 



THE SEASOisTiNG OF WOOD, 7 

Table II. — Average shrinlage of various species of wood. 



Species and locality. 



HAKDWOODS. 



Alder, red, Snohomish County, Wash . . . . 
Ash: 

Biltmore, Overton County, Tenn 

B lack- 
On tonagon, Mich 

Marathon County, Wis 

Blue, Bourbon County, Ky 

Green- 
Richland Parish, La 

, New Madrid County, Mo 

Oregon, Lane County, Oreg 

Pumpkin, New Madrid Countv, Mo. . 
White- 
Stone County, Ark 

Oswego County, N. Y 

Pocahontas County, W. Va 

Aspen: 

Rusk County, Wis 

Largetooth, Sauk Comity, Wis 

Bass wood: 

Potter County, Pa 

Marathon County, Wis 

Beech: 

Hendricks County, Ind 

Potter County, Pa 

Birch: 

Paper, Rusk County, Wis 

Sweet, Potter County 

Yellow- 
Potter County, Pa 

Marathon County, Wis 

Buckeye, yellow, Sevier "County, Tenn. . . 
Buckthorn, cascara, Lane County, Oreg. . 
Butternut: 

Sauk County, Wis 

Sevier County, Tenn 

Cherry: 

Biack, Potter County, Pa 

Wild red, Sevier Coiinty, Tenn 

Chestnut: 

Baltimore Coimty, Md 

Sevier County, Term 

Chinquapin, western, Lane Coimty, Oreg. 
Cottonwood : 

Pemiscot County, Mo 

Black, Snohomish County, Wash 

Cucumber tree, Sevier Comity, Tenn 

Dogwood : 

Flowering. Sevier County, Tenn 

Western, Lane Comity, Oreg 

Elder, pale, Douglas County, Oreg 

Elm: 

Cork- 
Marathon Coimtv, Wis 

Rusk County, Wis 

Slippery— 

Hendricks County, Ind 

Sauk County, Wis 

White- 
Potter County, Pa 

Marathon County, Wis 



No figures available. 



Shrinkage from a green to an 
oven-diy condition (per 
cent of dimensions when 
green). 



12.6 
12. C 



10.4 
IS. 9 
13.2 



15.5 
13.4 



(■) 



0) 

4, 



^6.2 



(») 



0) 



(') 




(') 






7.6 



0) 



0) 



8.1 



(') 



9.5 



Note.— Oven-dry means entirely free from water. The shrinkage from a green to a kiln-dry condition 
(8 per cent moisture) is generally about 75 percent of the shrinkage to anovon-dry condition, the shrink- 
age from a green to an air-dry condition (12 to 15 per cent moisture) is generally about 50 per cent of the 
shrinkage to an oven-dry condition. 



BULLETIN 552, U. S. BETAETMeNT OP AGEICFLTLTRE. 
Table II. — Average shrinkage of various species of wood — Continued. 



Species and locality. 




Shrinkage from a green to an 
oven-diy condition (r>er 
cent of dimensions wlien 
green). 



HARDWOODS— continued. 
Gum: 

Black, Sevier County, Tenn 

Blue, Alameda Comity, Cal 

Red- 
New Jfadrid County, Mo 

Pemiscot County, Ato 

Hackberry: 

Hendricks County, Ind 

Sauk County, Wis 

Haw, pear, Sauk County, Wis 

Hickory: 

Shellbark— 

Sardis, Miss 

Napoleon, Ohio 

Bitterriut, Napoleon, Ohio 

Mockernut— 

Sardis, Miss 

Chester County, Pa 

Webster County, W. Va 

Nutmeg, Sardis, Miss 

Pignut- 
Sard is. Miss 

Napoleon, Ohio 

Chester County, Pa 

Web!<tcr County, W . Va 

Shagbark — 

Sardis, f tiss 

Napoleon, Ohio 

Chester County, Pa 

Webster County, W . Va 

Water, Sardis, ftCiss; 

Holly, American, Sevier County, Tenn 

Hornbeam, Rusk County, Wis 

Laurel: 

California. Doupjlas County, Oreg 

Moimiain, Sevier County, Term 

Locust: 

Black, Sevier County, Tenn 

Honey, Hendricks Coimty, Ind 

Madrona:" 

Hiitte Coimty. Cal 

l^ouglas County, Oreg 

Maple: 

Broadleaf, Snohomish County, Wash 

Retl- 

Marathon County, Wis 

Potter County, Pa 

Silyer, Sauk Coimty, Wis 

Sugar— 

Hendricks Coimty, Ind 

Potter County, Pa 

Marathon Count>', W is 

Magnolia (evergreen ), Winn Parish, La 

Oak, bur, Sauk County, Wis 

California black — 

Rutte ComUy, Cal 

Douglas County, Oreg 

Canyon live, Butte County, Cal 

Chestnut, Sevier County, Tenn 

Cow, Wiim Parish, La 

Laurel, Winn Parish, La 

Pacific post, Douglas Coimty, Oreg 

Post- 
Stone County, Ark 

Winn Parisli, La 



13.9 


4.4 


22.5 


7.6 


(') 




(') 


14.0 


4.2 


13.8 


4.9 


(') 


(n 


1.7.6 


7.4 


20.9 


7.9 


0) 


(') 


16.5 


6.9 


18.9 


8.4 


(>) 


^'\ 


(') 


0) 


15.0 


5.6 


15.3 


6.3 


16.9 


6.8 


21.2 


8.5 


16.0 


6.5 


18.4 


7.9 


(») 


(>) 


15.5 


6.5 


(') 


(') 


16.2 


4.5 


(') 


8.2 


(•) 


0) 


14.4 


5.6 


9.8 


4.4 


8.6 


0) 


16.2 


5.1 


17.6 


5.5 


11.6 


3.7 


(') 


(') 


12.5 


3.8 


12.0 


3.0 


14.3 


4.9 


14.7 


4.8 


(1) 


(») 


12.3 


5.4 


12.7 


4.4 


13.6 


4.1 


(') 


0) 


16.2 


8.0 


16.7 


5.5 


19.4 


5.9 


19.0 


3.9 


(') 


0) 


16.0 


5.7 


16.5 


5.2 



1 No figures available. 



THE SEASONING OF WOOD. 
Table II. — Average shrinkage of various speaes of iroor^/— Continued. 



9 



Species and locality. 



Shrinkage from a green to an 
oven-dry condition (per 
cent of dimensions when 
green). 



HARDWooDs^tontinued. 
O^k— Continued. 
Red- 
stone County, Ark 

Hendricks County, Ind 

Richland Parish, I/a 

Sevier County, Tenn 

Spanish — 

Lowland, Winn Parish, La 

Highland, Winn Parish, La 

Swamp white, Hendricks Coiuity, Ind 

Tanbark, Willits, Cal ". . 

Water, Winn Parish, La 

White- 
Stone County, Ark 

Hendricks County, Lnd 

Richland Parish,"La 

Winn I'arish, La 

Willow, Winn Parish, La 

Yellow^ 

Stone County, Ark 

Marathon County, Wis 

Osage orange, Morgan County, Ind 

Poplar, yellow, Sevier County, Tenn 

Rhododendron, great, Sevier County, Tenn 

Sassafras, Sevier Coimty, Tenn 

Serviceberry , Sevier County , Tenn 

Silverbell tree, Sevier County, Tenn 

Sourwood, Sevier County, Term 

Sumach, staghorn, Sauk Coimty, Wis 

Sycamore: 

Hendricks County, Ind 

Sevier County, Term 

Tupelo. St. John the Baptist Parish, La 

Umbrella, Fraser, Sevier County, Tenn 

Walnut, black, Kentuckv 

Willow: 

Western black, Dduglas County, Oreg 

Black, Sauk County, Wis 

Witch hazel, Sevier County, Tenn 

C0N1FER,S. 

Cedar: 

Incense — 

Lane County, Oreg ^..^ 

Weed, Cal 

Port Orford , Douglas County, Oreg 

Western red— 

Missoula County, Mont 

Snohomish County, Wash 

White, Shawano County, Wis 

Cypress: 

Bald, St. .John the Baptist Parish, La 

Yellow, Lane County, Oreg 

Douglas fir: 

Plumas County, Cal 

Humboldt County, Cal 

.Tohnson County, "Wyo 

Lane County, Oreg 

Chehalis County, Wash 

Lewis County, Wash 

Washington and Oregon 

Missoula County, Mont 

Fir: 

Amabilis — 

Dee, Oreg 

Snohomish Coimty, Wash 

' No figures available 
S7732°— Bull. 552—17 2 




14. 
13. 

(') 
15. 

16. 
16. 
17. 
(') 
16. 

15. 
14. 
16. 
16. 

18. 

14. 

(0 

8. 

11. 

16. 

II). 

18. 

12. 

15. 
(•) 

1.3. 
14. 
12. 
13. 
11. 

13. 
13. 
18. 



7.6 
8.6 
7.0 

11.5 
(') 

11.7 

(') 
10.9 
13.2 

12.5 
12.3 

(') 

(') 



(') 
14.1 



0) 



4.2 
3.7 



3.7 

5.2 

4.5 
5.5 



(') 



6.2 
4.9 
4.8 
6.4 
5.0 



(•) 



(') 



4.1 

6.3 
4.0 
6.7 
3.8 
6.3 



5.0 
.5.2 

4.4 
4.4 
5.2 

2.9 
2.2 



(') 



2.5 
2.5 
2.1 



(') 



(') 



3.7 

5.7 
4.4 
6.0 



(') 
(>) 



(') 



4.5 



(•) 



(') 



8.3 
8. J 

8.3 

10.8 
8.7 
10.0 






8.3 
9.0 
9.2 
9.5 
9.6 

9.7 



(') 



6.9 
8.7 
6.2 
10.8 
7.6 
8.9 



(') 



7.3 
7.9 
7.9 
12.5 
7.1 

9.0 

8.2 



(') 



4.6 
5.6 

4.7 

6.0 



(') 



(') 
(') 



0) 



6.9 
I 

6.6 
7.6 
7.4 
8.3 



10.0 



10 BULLETIN 552, U. S. DEPARTMENT OF AGRICULTURE. 

Table II. — Average shrinkage of various species of wood — Continued. 



Species and locality. 



CONIFERS— continued. 



Fir— Continued. 

Alpine, Grand County, Colo 

Balsam, Eusk County, Wis 

Grand — 

Missoula County, Mont 

Douglas County, Oreg 

Noble, Multnomah County. Oreg 

White, Madera County, Cal 

Hemlock: 

Sevier County, Tenn 

Marathon County, Wis 

Mountain, Missoula County, Mont 

Western— 

Chehalis County, Wash 

Grays Harbor and Buckley, Wash. . . 
Larchj western: 

Missoula County, Mont 

Stevens County, Wash 

Pine: 

Jack, Barron County, Wis 

Jeffrey, Plumas County, Cal , 

Loblolly, Nassau County, Fla 

Lodgepoie — 

Grand County, Colo 

Gallatin County, Mont 

Granite County, Mont 

Jefferson County, Mont 

Johnson County, Wyo 

Longleaf — 

Nassau County, Fla 

Bogalusa, La 

Lake Charles, La 

Tangipahoa Parish, La 

Hattiesburg, Miss 

Pitch, Sevier County, Tenn 

Pond, Nassau County, Fla 

Norway, Shawano County, Wis 

Shortleaf— 

Malvern, Ark 

Bogalusa, La 

Slash, Nassau County, Fla 

Sugar, Madera County, Cal 

Table- mountaui, Sevier County, Tenn. . 
Western white, Missoula County, Mont. 
Western yellow— 

Coconino County, Ariz 

Madera County, (.'al 

Donglas Coimty , Colo 

Missoula County , Mont 

White, Shawano County, Wis 

Redwood: 

Humboldt Co^unty , Cal 

Mendocino County, Cal 

Spruce: 

Engelmann — 

San Miguel County, Colo 

Grand County, Colo 

Red- 
Coos Coimty, N. H 

Sevier County, Tenn 

Sitka, Chehalis County, Wash 

White- 
Coos County, N. H 

Rusk County, Wis 

Tamarack, Shawano County, Wis 

Yew, western, Snohomish County, Wash. . . 



Shrinkage from a green to an 
oven-dry condition (per 
cent of dimensions when 
green). 



Volume. 

1 






9.0 
10.8 

10.9 
(') 
13.6 
10.2 

n.6 

9.2 
10.8 

(') 
(') 

13.2 

(') 

10.4 
9.9 
12.6 

11.3 
11.9 
11.8 
12.0 
10.1 

12.2 
(') 
12.8 
12.8 
11.0 
11.7 
11.2 
1L5 

0) 

(') 

12.7 
8.4 
10.9 
11.5 

9.2 

n.5 

9.9 
9.3 

7.8 



10.3 
10.5 

(') 

n.8 

11.2 

14.8 
13.6 
•9.7 



2.5 

2.8 



3.5 



(') 



4.9 
3.4 



2.3 
4.4 



(0 



3.4 
4.4 
5.5 

4.2 
4.6 
5.0 
6.0 
3.6 



Tangen- 
tial. 



(') 



5.1 



5.4 
6.0 
4.8 
4.8 
5.1 
4.6 



f.! 



5.9 
2.9 
3.4 
4.1 

4.1 

4.9 
3.8 
3.5 
2.2 



u 



3.0 
3.7 



(■) 



0) 



3.7 
3.7 
4.0 



' No figures available. 



THE SEASONING OF WOOD. H 

HOW WOOD MAY BE INJURED IN SEASONING. 

CHECKING. 

Checking is caused by unequal shrinkage. If the outside of a piece 
of wood dries considerably faster than the inside, the surface in time 
will contract until it can no longer extend around the comparatively 
wet interior, and so will be torn apart in checks. Checks often are 
classified as end checks and face checks. End checking or spUtting 
during seasoning causes nearly as much loss as face checking. 

CASEHARDENING. 

Casehardening or surface hardening occurs when the surface of 
wood becomes set in a partially dry condition while the interior is 
still wet. This condition results from too rapid surface drying. 
If the interior of a casehardened piece of wood dries further, it tends 
to shrink, while the "set" condition of the surface tends to prevent 
it from doing so. As a result, stresses are set up in the piece. Plate I , 
figure 1, shows sections cut from casehardened boards, with a strip 
sawed from the center of each section. In A, the stresses cause the 
prongs to curve inward and bind on the saw. If the stresses are 
reheved by treatment with steam, as may be done sometimes, and 
the board dried a second time, the resawed prongs, as shown in B, 
will curve outward, owing to a reversal of the stresses. This is 
termed ''reverse casehardening." ^ 

Plate I, figure 2, shows the form taken by resawed pieces of kiln- 
dry boards steamed for different lengths of time. In No, 1 the 
prongs curve inward, owing to casehardening. No. 2 and No. 3 
also show a casehardened condition as indicated by the strips curving 
inward. In Nos. 4, 5, and 6 the casehardening has been eUminated 
by longer steaming and the resawed strips are straight. No. 7, which 
has been steamed stiU longer, shows a condition of " reverse case- 
hardening," in wliich the resawed strips curve outward. 

Sections cut as shown in Plate I may be used also to determine the 
distribution of moisture in lumber, whether casehardened or not. 
If not casehardened, such sections wiU curve inward as they dry 
if the lumber is wetter on the inside than on the surface, and outward 
if the reverse is the case. If the lumber is uniformly dry, the prongs 
wiU remain practically straight. 

HONEYCOMBING. 

Honeycombing or internal checking occurs in casehardened 
pieces when the interior continues to dry and the surface remains 
fixed. In such cases splits appear in the interior. Plate II, figure 1, 
shows examples of honeycombing in casehardened pieces. 

iFor further discussion see " Problems in Kiln-Drying Lumber," by H. D. Tiemann, Lumber World 
Review, Sept. 23, 191o, 



12 BULLETIN 552, U. S. DEPAETMENT OF AGKICULTUKE. 

WARPING. 

Warping or twisting in lumber is due to unequal shrinkage. Some 
woods are much more subject to warping than others. The trouble 
can be prevented to some extent by careful pihng, both during 
drying and afterward. Plate II, figure 2, shows badly warped 
pieces of lumber. 

COLLAPSE. 

In some woods, notably western red cedar and redwood, when 
the very wet wood is dried at a high temperature, depressions appear 
on the surface of the boards, presumably due to the coUapse of the 
plastic cell walls in certain places. If, however, the woods in ques- 
tion are heated above the boihng point while wet, the steam generated 
in the nonporous cells causes the wood to bulge on the surface. 
Plate III shows collapse and bulging, or ''explosion," as it is termed 
by the discoverer ol the phenomenon.* 

AIR-SEASONING. 

Though the use of dry kilns is increasing steadily, the bulk of our 
wood is still seasoned in the open air. If kept in the air long enough, 
the moisture content of the wood finally comes into equihbrium with 
that of the surrounding atmosphere, and the wood is said to be air- 
dried. The rate of drying varies, of course, with time of year, 
species of wood, size and form of piece, and method of piling. Certain 
of these factors may be controlled or utihzed in a way to hasten the 
drying process and lessen the hkehhood of defects appearing in the 
material. 

CROSSTIES, POLES, AND SAWED TIMBERS. 

The data in figures 2 to 12, inclusive, collected by the For- 
est Service^ in various parts of the country, show the rate at 
which crossties, poles, and sawed timbers of several species lose mois- 
ture when freely exposed to the atmosphere. In some cases it was 
not possible to weigh the pieces for several days after they were cut. 
FresUy cut timber loses weight very rapidly in warm dry weather. 
Ties in some species lose 10 pounds in 24 hours. The rates of season- 
ing of the various species may be compared by the general trend of 
the curves. When the curves reach a horizontal position, the ma- 
terial may be said to be air-dry, unless this happens at a time of 
year very unfavorable for seasoning. 

The ties were seasoned in piles of 50 each, and were exposed with- 
out cover. The ties on the top of each pile, however, were placed 
close together and served as a rough roof. The curves are platted 

1 H. D. Tiemann, in charge Section of Timber Physics, Forest Products Laboratory of Forest Service. 

2 See " The Air Seasoning of Timber," by W. H. Kempfer, Forest Service, in Bui. 161 of the American 
Railway Engineering Association. 



Bui. 552, U. S. Dept. of Agriculture. 



Plate I. 




Fig. 1 .—Sections Cut from Casehardened Boards. 




Fig. 2.— Resawed Sections Cut from Casehardened Red Gum Boards Steamed 
for Different Lengths of Time after being Kiln Dried. 

(1) No final steaming; (2) and (3) 18 minutes final steamins;; (4). (5), and (6) 36 minutes final steam- 
ing; (7) 3 hours final steaming. 



Bui. 552, U. S. Dept. of Agriculture. 



Plate II. 




Fig. 1.— Honeycombed Oak Timbers, the Result of Casehardening. 




FiQ. 2.— Badly Warped Boards. 

Tlie trouble here is due to a poor arrangement of stickers and to the piling together of boards of 

unequal length. 



THE SEASONING OF WOOD. 



13 







































































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2 4 6 8 

TIME SEASONING -MONTHS 




2 4" 

TIME SEASONING' 



Fig. 2. — Southwestern ties, (o) Seasoning of ties at Pecos, N. Mex.; cut in January and February; 
(b) seasoning of ties at Pecos, N. Mex. ; cut in August, September, and October. (Black and red pine 
are local names for western yellow pine; black pine refers to young trees and red pine to mature trees). 




6 8 10 

TIME SEASONINO- MONTHS 
Fig. 3. — Northwestern ties. Seasoning of Indgepole pine ties at Bozeman, Mont., Douglas fir at Sandpoint, 
Idaho (curves U and 12), Pasco, Wash, (curve 15), and Tacoina, Wash, (curve 16); and western larch at 
Sandpoint, Idaho, cut in January and February; Tacoma ties in December and January. 



14 



BULLETIN" 552, U. S. DEPARTMENT OF AGRICULTURE. 















































































200 






























































I/C- HEMLOCK- UNPEELED 






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TIME SEASONING -MONTHS 



Fig. 4.— Seasoning of hemlock ties at Escanaba, Mich.; cut in August and September. 



240 




























































220 
200 
































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TIME SEASONING -MONTHS 
Fig. 5. — Seasoning of ties at Silsbee, Tex., and Ackerman, Miss.; cut in January and February. 



THE SEASONING OF WOOD. 



15 




6 8 10 la 

TIME SEASONING -MONTHS 

Fig. 6. — Seasoning of hardwood ties in Southern States; cut in October, November, and December. 



t^ae 



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TIME SEASONING -MONTHS 

FiQ. 7.— Seasoning of southern white cedar poles at Wilmington, N. C. 




6 8 10 12 14 i§ 

TIME SEASONING - MONTHS 

Fig. 8.— Seasoning of western red cedar poles at Wilmington, Cal, 



16 



BULLETIN 552^ U. S. DEPARTMENT OF AGRICULTURE. 



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6 8 10 12 14 16 13 

TIME SEASONING - MONTHS 
Fig. 9.— Seasoning of chestnut poles at Thorndale, Pa. 



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riM£ SEASONING - MONTHS 
FiQ. 10.— Seasoning of northern white cedar poles at Escanaba, Mich. 




TIME 8IAS0HIN6 - MONTHS 

Fig. 11.— Seasoning of Douglas fir timbers at Eugene, Oreg. 



Bui. 552, U. S. Dept. of Agriculture. 



Plate 




Collapse and Bulge in Very Wet Lumber Dried at a High Temperature. 



Bui. 552, U. S. Dept. of Agriculture. 



Pl^te IV. 




Fig. 1.— Cement as Foundation Material. 
These blocks extend 2 feet above and 2 feet into the ground. 




FiQ. 2.— Another Type of Permanent Foundation. 

Steel rails are embedded in cement piers. The himlier is fully 2\ feet above the ground, insuring 

excellent ventilation. 



THE SEASONING OF WOOD. 



lY 



from the average weight of the ties. The weight per unit volume 
could not be used, as in many cases the volumes of the ties were not 



50 












































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LONGLEAF PINE 


















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TIME SEASONING -MONTH 
Fig. 12.— Seasoning of longleaf pine timbers at Madison, AVis. 

obtained. Tlie poles were seasoned on skids in the open. The 
sawed timbers were seasoned in open piles under shelter. 



Sawed lumber generally is dried by being piled in stacks with air 
spaces between the boards. In forming the stacks the boards usually 
are laid flat, with strips called stickers between courses or layers. 







Fiu. i3.— i^umoer piiea siaewise on cement ana metal rounaatlona. 



18 



BULLETIN 552, U. S. DEPARTMENT OP AGRICULTURE. 



A space also is left between each board in a layer and the adjacent 
board to provide for the circulation of air throughout the stack. 
Flat or horizontal piUng may be of two kinds: (a) With the ends of 
the boards toward the alley — endwise pihng, and (b) with the sides 
toward the alley — sidewise piling. Figures 13 and 14 illustrate the 
two methods. The stacks are arranged to slope from front to rear, 




Fig. 14. — Lumber piled lengthwise on wooden foundation. 

and to lean forward so that water dripping from the top falls to the 
ground without trickhng down over the courses below. With either 
method of piling the stacks should be so located in the yard that the 
prevaihng winds blow through them rather than against the ends. 

Most lumber manufacturers and dealers use the endwise method 
of piling. A number, however, have adopted the sidewise method, 







— ^'==3i=:l^«^J==fl=3t^ ^^^^ 



Fio. 15. — Method of providing drainage under lumber piles. 

which has certain advantages in the matter of air circulation. In 
endwise pihng the stickers obstruct the passage of air from back to 
front of a course, while in sidewise pihng the passages from front to 
rear are clear. Water wliich forces its way into the pile is more 
efficiently drained in sidewise piling, and the likelihood of sticker rot 
and discoloration due to the accumulation of moisture, dust, and dirt 
against the stickers is lessened. 



THE SEASONING OF WOOD. 19 

The bottom boards in a stack rest on skids, which in turn rest on 
foundations, preferably of stone, cement, or metal. Pieces contain- 
ing rot should never be used for foundation timbers or skids, or allowed 
to remain in the pile. The vicinity of the pile should be kept clear 
of weeds. 

The use of cement and metal foundations is especially feasible in 
retail lumber yards and in those maintained by wood-using factories. 
In retail yards, where economy in space often is the essential thing, 
the piles are high and a particular space usually is allotted to each 
class or species of lumber. In factory yards lumber often is held for 
a number of years before being used. In such cases the frequent 
renewal of wooden foundations under lumber piles entails consider- 
able expenditure of time and money, to say nothing of the danger 
of infecting lumber by bringing it in contact with partly rotted 
foundation timbers. For these reasons foundations of a more per- 
manent character are constantly growing in favor in retail and factory 
yards. Plate IV shows foundations of this kind. 

Sawmill yards, on the other hand, often contain several million 
feet of material and cover several acres. Lumber coming from the 
saw generally is piled wherever most convenient, provided it is placed at 
the distance from the mill required by insurance companies. Economy 
in storage space generally is not essential, and piles of the same spe- 
cies and kind of lumber are Ukely to be found in a number of different 
sections of the yard. In addition, the stock is constantly being turned 
over, thus giving an opportunity to renew the foundation timbers at 
comparatively small expense. A number of large lumber companies, 
however, have adopted cement as a foundation material. 

Lumber-storage yards need to be reasonably well drained, or at 
least the contour of the ground should be such that water will not 
•stand imder the stacks after a storm. Otherwise decay is apt to get 
a start and spread throughout the pile. Where the ground offers 
but poor natural drainage f acihties, some artificial system of drainage 
usually is employed. Figure 15 shows the system used in the yards 
of two large lumber companies in the southern hardwood region. 
This arrangement not only prevents the collection of rain water 
under the lumber piles, but also gives the required slope to the stack, 
which on level ground has to be secured by building up the founda- 
tions/ 

A top dressing of cinders has been found satisfactory in some 
storage yards. 

The following set of rules for piling lumber covers the more im- 
portant points to be observed in the construction of foundations, 
shape of stack, arrangement of stickers, etc. : 

» Humphrey, C. J. Timber Storage Conditions in the Eastern and Southern States, with reference to 
Decay Problems. U. S. Dept. Agr. Bui. 510. 1917. 



20 BULLETIN 552, U. S. DEPARTMENT OF AGRICULTURE^ 

RULES FOR PILING LUMBER. 

1. Foundations (Endwise or Sidewise Piling). 

(a) The foundations should be strong, solid, and durable. 

(6) The top of each foundation should be level, and from front to back the top 
surface of the parallel skids should be in alignment, so that the lumber to be piled 
will bear equally upon each one. 

(c) The front foundation should be raised above the second, and the second above 
the third, to allow a slant in the stack of 1 inch to every foot. 

{d) The foundations should be spaced not over 4 feet apart, except for heavy planks 
and timbers. 

(e) The front foundation should be of sufficient height to provide space for free 
circulation of air under all parts of the pile. 

2. Lumber (Endwise Piling). 

(a) Skids, preferably 2 by 4 inches, should be laid on top of the foimdations. 

(6) Boards of equal length should be piled together. 

(c) The ends of the boards should rest upon the front and rear skids. 

(rf) A space of approximately three-fourths inch should be left between boards in 
the same layer. 

(e) Lumber piled in the open should have the front ends of boards in each layer 
slightly protruding beyond the end of the layer beneath, to give a forward pitch to 
the stack. 

3. Lumber (Sidewise Piling). 

(a) Skids, preferably 4 by 6 inches, should be placed across the foundations at 
about 4-foot intervals. The number of skids depends upon the thickness of the 
lumber. 

(6) Boards of equal length should be piled together. 

(c) The boards should be placed on the skids, with about three-fourths inch between 
boards in the samo layer. 

(c?) Lumber piled in the open should have the front board in each layer project 
slightly beyond the board in the layer beneath, to provide a forward pitch to the stack. 

4. Stickers (Endwise or Sidewise Piling), 

(o) Stickers should be of uniform thickness, preferably seven-eighths inch for 
1-inch lumber and li inches for 2-inch lumber. Their length should be a few inches 
in excess of the width of the pile. 

(6) Stickers should be placed upon the layer of boards immediately over the skids 
and kept in alignment parallel to the ftont of the piles. 

(c) The front and rear stickers should be flush with, or protrude beyond, the ends 
of the boards. 

5. Roof Protection (Endwise or Sidewise Piling). 

Cover boards, as a roof protection, should be laid on the top of the pile, extending 
a few inches beyond the front and rear ends of the stack, 

6. Spacing Stacks (Endwise or Sidewise Piling). 

Space between the piles should not be less than 2 feet; 4 or 5 feet is preferred if 
yardage conditions permit. 



Bui. 552, U. S. Dept. of Agriculture. 



Plate V. 




Fig. 1.— Sun Shields Used to Reduce Checking in Thick Red Oak Timber. 




Fig. 2.— Lumber Piled so as to Form "Chimney" or Flue near Center of Stack 
FROM Bottom to Top. 



Bui. 552, U. S. Dept. of Agriculture. 



Plate VI. 




FiQ. 1.— Pole Drying Yellow Poplar Lumber. 




Fi3. 2.— Pole Framework Used to Dry Yellow Poplar, Basswood, 
AND Cottonwood before Placing It in a Stuck Pile. 



Bui. 552, U. S. Deot. of Agriculture. 



Plate VII. 




H a 

2 2 



5 a 

> '^ 

> '3 

< ^ 

CO ft 



Bui. 552, U. S. Dept. of Agriculture. 



Plate VIII. 




THE SEASONING OF WOOD. 21 

7. Dimensions op Stack (Endwise or Sidewise Piling). 

The customary width of stacks is from 8 to 16 feet. The height is governed by 
the size and character of the lumber and by the methods of moving it. 

8. Treated Ends (Endwise or Sidewise Piling). 

The ends of lumber 2i inches thick or over, unless of the lower grades, should receive 
a brush treatment of paint or some liquid filler. 

The rules just given are based on information obtained through 
field investigations and from lumber manufacturers and wholesale 
and retail dealers, and accord with the best lumber-pihng practice 
in general commercial use. Certain species of wood, however, re- 
quire particular care in air-drying, and in this case shght variations 
from the rules are necessary in order to secure the best results. Some 
lumbermen in the South, for example, find that thick red oak checks 
badly on the ends, and in au--drying such stock have adopted the 
scheme of protecting it with sun shields, as shown in Plate V, figure 1, 
which they claim reduces end-checking to a minimum. 

Mills cutting red gum formerly experienced diflSculty in drymg 
the lumber, on account of its tendency to warp. This objection, 
however, has been largely overcome by the exercise of care in sea- 
sonmg. In erecting a pHe of gum lumber, stickers are placed every 
2 feet apart, some lumbermen claiming that 18 inches in none too 
close to obtain the best results. Another scheme in more or less 
general use among gum-lumber manufacturers is to construct the 
pHe so as to have a flue or "chimney" in its center, thus providing 
ample air circulation vertically through the stack, as shown in 
Plate V, figure 2. 

Green cottonwood, basswood, and yellow poplar lumber is hkely to 
stam badly when piled. Accordingly, a number of lumbermen either 
end-dry the material or pole-dry it for a week or two and then place 
It in a 'stuck" pile. In end^rying, the boards are stood up on end 
edge to edge, under a speciaUy built shed, with stickers arranged 
horizontally one above the other at specified distances. Such a 
pile presents exactly the appearance of a regular lengthwise pile of 
lumber set up on end. 

Plate VI,figure 1, shows a quantity of yellowpoplar lumber beingpole- 
dned, while Plate VI, figure 2, shows the frame used for the purpose 

Hickory and ash lumber frequently check badly when air-dried 
Lumbermen m the southern hardwood region have found that these 
checks wiU close up entirely if the lumber is first stuck-piled for 
6 to 8 months and then bulk-piled and protected by good covering 
preferably sheds. ^' 

Plates yil and VIII show lumber piles in yards where careful 
attention has been given to the matter of pihng and yard arrange- 



22 BULLETIN 552^ U. S. DEPARTMENT OF AGEICULTURE. 

KILN-DRYING. 

Lumber is kiln-dried when there is need for seasoning it quickly, 
or when the manufacturer does not wish to carry large stocks in his 
yard. A kiln is used also to further dry partially au'-seasoned or 
even fully air-seasoned material, for special uses. 

The main problem in kihi-drying lumber is to prevent the moisture 
from evaporating from the surface of the pieces faster than it is 
brought to the surface from the interior. When this happens the 
sm'face becomes considerably drier than the interior and begins to 
shrink. If the difference in moisture content is sufficient, the sur- 
face portion opens up in checks. 

The evaporation from the surface of wood in a kiln can be controlled 
to a large degree by regulating the humidity, temperature, and 
amount of air passing over the wood; and a correctly designed kiln, 
especially one for drying the more difficult woods, should be con- 
structed and equipped in a way to insure this regulation. 

A dry kihi may consist simply of a box in w^hich lumber can be 
heated, or of a good-sized building or group of buildings (battery) 
containing steam pipes, condensers, sprays, and various air pas- 
sages capable of adjustment to regulate the amount of ventilation. 
The elaborateness of the kiln depends, of course, mainly upon the 
value of the lumber that is to be dried. For lumber worth SI 00 
per 1,000 board feet, it obviously will pay to use more careful dry- 
ing methods than in handling material valued at $20 or $25 per 
1,000 board feet. 

TYPES OF KILNS. 

Kilns for drying lumber may be divided into two general classes 
(fig. 16): (a) compartment kilns, and (6) progressive kihis. In 
compartment kihis the conditions are changed during the drying 
process, and all lumber in the kiln is dried at one time. The condi- 
tions at any time diuing drying are uniform throughout the whole 
kiln. In a progressive kihi conditions at one end differ from those 
at the other, and the lumber is dried progressively by being passed 
through the kihi. Compartment kilns are used w^hen it is desired 
to dry lumber of various sizes and species, while progressive kilns 
arc used where uniform stock is handled. 

The methods of operation generally used in lumber kilns are: (a) 
natural ventilation, (h) condensing, and (c) superheated steam. 

In kilns operating by natural ventilation, the humidity or dampness 
is controlled by the use of escaping steam and evaporated moisture. 
Circulation in progressive kilns is largely longitudinal and in compart- 
ment kilns transveree. Moist air is allowed to escape from the kiln. 

In condensing kilns the humidity is controlled by recirculating the 
air, which has taken up water from the lumber, across water pipes 



THE SEASONING OF WOOD. 



23 




Fig. 16.— TypesofkUns, 



24 BULLETIN 552, U. S. DEPARTMENT OF AGEICULTUEE. 




INCLINED FILES 




£DGC fILCS 
Fig. 17.— Methods of piling lumber for kiln drying. 



THE SEASONING OF WOOD. 25 

or through water sprays. The temperature of the pipes or sprays 
governs the amount of water that condenses from the air, and con- 
sequently regulates the humidity of the air when reheated before 
being passed over the lumber again. The circulation of air may be 
either natural or forced. Condensing kilns are generally of the com- 
partment type. 

Kilns operating with superheated steam are used only where the 
species to be dried are not injured by high temperatures, and where 
fast drymg is essential. 

Lumber may be piled on the trucks which carry it into the kiln in 
any one of three ways (fig. 17): (a) flat or horizontal, (h) edge or 
vertical, and (a) incHned. Flat piUng is best for longitudinal circu- 
lation. It is not so well adapted for transverse circulation, and is not 
economical for downward circulation. Vertical piluig mcreases the 
truck capacity, as there are no vertical spaces between the boards. 
Probably it is the best method for downward or any fast circulation. 
Provision has to be made, however, for keeping the boards in place in 
the stack. Inchned piling allows for a definite movement of air either 
downward or upward (forced draft). It is an improvement, as 
regards circulation, over horizontal or flat piling. 

Some kiln operators usmg the flat or horizontal method of piling 
report excellent results from the construction of a V-shaped, openmg 
in the center «f the truck pile. Such openings are from 2^ to 3 feet 
wide at their base, and from 3^ to 4 feet high. Wliere this practice 
is followed it is customary also to place the boards in the layers closer 
together as the top of the stack is reached, to force greater lateral 
circulation. 

In loading lumber on kiln trucks by any one of the three methods 
mentioned, the stickers should be of a uniform thickness and arranged 
in the piles in ahgnment. 

It is advisable also not to attempt to dry various thicknesses of 
lumber together. Thick lumber takes longer to dry than thin 
lumber, and when different thicknesses are mixed the operation has 
to be governed by the thick stock, to the possible detriment, or at 
least the unnecessarily long drying, of the thin stock. 

PRELIMINARY TREATMENTS. 

Lumber to be kiln-dried is sometunes steamed in a separate com- 
partment before being placed in the kihi proper, especially where the 
kiln is not designed for moist-air treatment. In transferring lumber 
from a compartment for prelimmary treatment to the kihi proper, 
every care must be used to avoid a sudden change in humidity. The 
object of the steaming is to heat the lumber and thus make easier 
the transmission of moisture from the interior to the surface, and also 
to moisten the surface in case it has become casehardened or "'set" 



26 BULLETIN 552, U. S. DEPARTMENT OF AGRICULTURE. 

during partial air drying. Other effects, also, are produced, which to 
a greater or less extent change the properties of the wood. The 
' ' sap ' in the wood is changed by ' ' cooking," as indicated by a darken- 
ing of the wood, the degree of coloring depending upon the tempera- 
ture and duration of the process. Other changes of a chemical 
nature apparently also take place m the wood durmg steaming. 

The pressure and duration of steaming desirable in kiln-drying 
have not yet been thoroughly worked out. Durations of from 5 
mmutes to 24 hours or longer, and pressures rangmg from atmospheric 
to 50 pounds gauge, have been used in practice. The higher the 
pressure the greater is the effect produced, and the longer the time 
the more thoroughly the treatment penetrates the wood. Experi- 
ments have shown that a pressure slightly above atmospheric for 24 
hours will shghtly darken 2-inch maple clear through, and a pressure 
of 40 pounds will turn oak and probably other hardwoods almost 
black. Even where the strength of the wood is not the primary 
consideration, it probably is not safe to exceed 15 pounds gauge 
pressure (250° F.), except for special purposes. 

THE PROCESS OF DRYING. 

After the wood has been heated thoroughly in a humid atmos- 
phere, either in the kiln proper or in a separate compartment, it is 
ready to have the moisture removed by evaporation from the sur- 
face. In kdn-drying uniform circulation apparently is the most 
important thing to be secured. The fact that air when it enters the 
drying chamber wiU be cooled, and therefore will tend to fall, should 
govern the method of pihng and the direction of circulation.^ This 
means that the air should be allowed and assisted to pass downward 
through the pile, either by entering at the top of the pile or by an 
adaptation of this principle to other methods of piling. The rate of 
evaporation may be controlled best by regulating the amount of 
moisture in the air (relative humidity) circulating about the lumber 
in the kiln; it should not be controlled by reducing the air circula- 
tion, since a large circulation is needed at all times to supply the 
necessary heat. Air at 100 per cent relative humidity contains aU 
the water it can carry and has no effect in drying wood. If, how- 
ever, the humidity is reduced to 90 per cent and the air then passed 
through a pile of wet lumber, the air can take up a certain amount 
of moisture. If drying does not progress rapidly enough with the cir- 
culating air at 90 per cent humidity, it may be reduced still further. 
This may be accompUshed by ventilation, by condensers, by water 
or steam sprays, or in a number of other ways. Any well-made kiln 
which will allow the control of the humidity, temperature, and cir- 

1 See " The Circulation in Dry Kilns," by H. D. Tiemann, Lumber World Review, May 10 and June 10, 
1910. 



THE SEASONING OF WOOD. 



27 



culation of the air passing over the lumber should give satisfactory- 
results. If checking begins during the drying process, the humidity 
should be increased until it stops. Steam jets in a kiln are often useful 
for this purpose. In changing the humidity the circulation should 
not be reduced. A large body of 
moving air is necessary in order 
to keep a uniform temperature 
clear through to the center of 
each piece of wood in the pile and 
at the same time supply the heat 
required for evaporation. If suf- 
ficient circulation is not secured, 
the supply of heat for both pur- 
poses will be lacking and J;he 
material will not dry uniformly. 
Figure 18 shows the conditions 
in a kiln during a run with refer- 
ence to temperature, humidity, 
and moisture in the wood. It 
will be noted that the humidity 
is kept high at first and lowered 
gradually. The temperature is 
held at a certain level for some 
time and then raised. The mois- 
ture is lowered gradually to a 
final condition of less than 5 per 
cent. 

The maximum rate of drying 
at a given temperature is reached 
when moisture is evaporated 
from the surface of the wood just 
as fast as it is transmitted from 
the interior. This rate is fixed 
by the rate of transmission of 
moisture within the wood and 
varies with different woods. 

The temperature of drying ap- 
parently influences the rate of 
transmission of moisture within 
the wood. The higher the temperature of the wood the more rapid is 
the rate of transmission of the moisture, and hence the rate at which 
the moisture may be evaporated safely. This, of course, apphes only 
to temperatures below those which might result in injury to the wood. 







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28 BULLETIN 552, U. S. DEPAETMENT OP AGRICULTUEE. 

Drying tends to render wood more or less brittle. Although the 
strength of wood increases with its degree of dryness, yet wood which 
has been dried and resoaked is less resilient than when green. There- 
fore, where strength is the prime consideration, it is preferable not 
to dry the wood beyond the degree at which it is to be used. The 
final stage of kiln-drying is generally conducted at a humidity some- 
what below the actual humidity that on long exposure would pro- 
duce the same average moisture condition. This is done in order 
to hasten the drying and to make it uniform throughout each piece. 



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